In the past it has been frequently suggested to have two farm vehicles travel on parallel paths, wherein the first vehicle (master) is steered by an operator or automatically and the second vehicle (slave) is automatically kept on a track of travel parallel next to or behind the first vehicle. Both vehicles can perform a cultivation operation or the second vehicle can serve as the transport vehicle for the crop harvested by the first vehicle. By way of example reference is made to the following publications: “Elektronik als Hofknecht (Electronics used as Court Servants)”, Landtechnik 3/2000, pgs. 256 f; G. Wallmann and H.-H. Harms, “Assistenzsystem zur Überladung landwirtschaftlicher Güter (Support System for transferring agricultural goods)”, Landtechnik 6/2002, pgs. 352 f; DE 197 05 842 A, DE 100 57 374 A, DE 100 64 860 A, DE 100 64 862 A, DE 102 24 939 A, EP 956 522 B, and JP 04 101 206 A.
At least two types of transmission have been suggested for transmitting steering information from the first to the second vehicle. On one hand, steering data in the form of speed and directional data can be transmitted from the first vehicle to the second vehicle (see DE 100 64 860 A and JP 04 101 206 A), but this type has the disadvantage that possible errors in the directional and speed specifications to the second vehicle add up over time to large errors in the position information so that parallel travel is not always guaranteed.
On the other hand, it is possible (see Wallmann, loc. cit., DE 100 64 862 A, DE 102 24 939 A and EP 956 522 B) to equip both vehicles with a satellite-supported position capturing system, especially with GPS receivers, and establish a data transmission connection between the two vehicles. In this type, the first vehicle informs the second vehicle of the respective position of the first vehicle. Based on that data and the output of the position capturing system of the second vehicle, the relative position is calculated by forming the difference between the two absolute positions based on which a steering and/or speed signal is generated for the second vehicle.
Satellite-supported position capturing systems such as GPS, Glonass or the future Galileo employ several satellites located in an earth orbit and equipped with atomic clocks, which emit the electromagnetic (radio) waves containing time and identity or location information, respectively. The corresponding receivers have to receive the signals of at least three satellites, respectively, in order to be able to determine the current position in space. If the current time is also to be determined, signals from four satellites must be received. Position sensing accuracy increases with the number of satellites. The accuracy of the position sensing systems is commonly improved through the reception of radio signals containing corrective data emitted by reference stations in known locations.
In the case of slave vehicles steered on the basis of position capturing systems, errors in the absolute positions occurring during the subtraction operation are eliminated as long as the same reception conditions exist for both receivers and they hence use signals of the same satellites. The relative position then is sufficiently accurate. If different reception conditions for the two receivers exist, caused for example by the shading of one satellite for the first receiver (e.g., on the edge of a field covered by trees where the vehicle travels) while the second vehicle travels further inward in the non-shaded center of the field, errors result in the relative position which can lead to undesirable erroneous steering processes of the second vehicle.
It has further been suggested to detect the quality of signals of satellite-supported position capturing system and if the quality is not sufficient for sufficiently accurate position sensing, to determine the position based on other means, such as radio waves (U.S. Pat. No. 5,999,126 A), sensors for detecting a crop boundary (DE 101 29 133 A, DE 101 29 135 A) or inertial navigation systems (EP 0 604 404 A, JP 04 134 212 A). These means, however, are only provided for a single vehicle and cannot solve the problem of different reception conditions of two receivers.
It is suggested in JP 04 174 389 A to equip a vehicle with two satellite antennas. For a single satellite the better suited antenna is selected, respectively, in order to obtain as accurate a position signal as possible. This as well cannot solve the problem of different reception conditions of receivers on two vehicles.